Metal food and beverage containers require an internal protective coating to ensure that the food or beverage product does not come into contact with metal surfaces of the containers. Contact of such corrosive food products as tomato juice and sauerkraut with the metal inner surface of a food or beverage container can lead to corrosion of the container with subsequent deterioration in the appearance and taste of the food product. Exterior container coatings may be applied to provide corrosion resistance to the metal container during sterilization and storage of a food product. The coatings may be applied by various application techniques, including electrodeposition from water dispersions of coating resins. Electrodeposition provides improved utilization of coating materials, lower volatile organic emissions, and, particularly, a more uniform coating.
Coatings for metal food containers, and particularly food-contacting coatings, must be substantially defect free and additionally should possess high resistance to a wide variety of foods and beverages. A major difficulty in obtaining defect-free films is the general tendency cf water based coatings to form blisters during oven baking. Another difficulty involves the failure of resin dispersions to coalesce into continuous films during the film drying and baking steps. The use of low molecular weight resins or very soft resins in container coating compositions tends to reduce the severity of defects and can provide substantially defect-free films. However, the soft or low molecular weight resins generally are not suitable for the protection of metal surfaces from aggressive foods and beverages.
As a consequence, water-based coatings have found only limited use as interior coatings for steel and aluminum food containers. Currently available water based coating materials are not completely satisfactory because they exhibit less than optimum resistance to aggressive foods such as sauerkraut and tomato products. However, the need for useful water-based metal container coating compositions continues, in large part because of the widely recognized need to reduce volatile organic emissions during the coating and curing procedures. Various prior art coating compositions have employed epoxy acrylates of the type described in U.S. patents 4,247,439 (Matthews, et al.), 4,480,058 (Ting, et al.), 4,482,673 (Brown, et al.), 4,442,246 (Brown, et al.), 4,446,258 (Chu, et al ), and 4,476,262 (Chu, et al.).
Electrocoating compositions commonly include polymers made from derivatives of methacrylic acid, acrylic acid, styrene, maleinized oils and epoxy esters. Compositional ranges of these materials in polymers approved for contact with food are set out in 21 CFR 175-300, and typical compositions are described in U.S. Pat. Nos. 4,308,121 (Hazan) and 3,939,051 (Anderson et al.).
U.S. Pat. Nos., 4,598,109 and 4,425,451 (both Sekmakas, et al.) refer to water-dispersible epoxy-phosphate ester polymer salts that are used in combination with aminoplast or phenoplast resins. These references disclose the reaction of an epoxy resin with less than a stoichiometric quantity of phosphoric acid to provide an epoxy phosphate ester. The remaining oxirane groups are then reacted with a volatile amine to provide a water-dispersible product lacking oxirane functionality. U.S. Pat. No. 4,461,857 (Sekmakas, et al.) shows a similar coating material which includes from 25-85% of a carboxyl functional, organic solvent soluble copolymer salt with a volatile amine. U.S. patent 4,397,970 (Campbell, et al.) describes an improved process for preparing epoxy resin/phosphoric acid reaction products which are manufactured through the use of an intermediate blocking agent. U.S. Pat. No. 4,164,487 (Martin) refers further to coatings containing water-thinable, base-neutralized phosphoric acid/polyether epoxide reaction products employing two or more epoxy resins.